In general, styrene-butadiene latex is widely used as a binder for bonding an inorganic pigment such as clay, calcium carbonate, aluminum hydroxide (Al(OH)3), and titanium oxide (TiO2) on a piece of paper. The paper thus obtained is referred to as coated paper and various coating properties and physical properties of coated paper will change depending on the type of pigments or binders.
It is fold crack and paper stiffness as highlight important physical properties of the coated paper. In general, when there is a need to retract the paper used for wrapping etc, paper is subjected to printing, ruling and folding process. At this time, the popping a phenomenon in which paper is torn due to the high mechanical pressure is referred to as fold crack. When fold crack occurs, appearance of printed materials is seriously damaged and product values are thus considerably deteriorated. When the deterioration is serious, mechanical strength of wrapping materials is also greatly deteriorated and problems associated with functions of wrapping materials are serious. The paper stiffness is also important to effectively protect the package contents and the paper stiffness generally tend to weaken, as fold crack decreases.
The factors that have the most greatest impact on the fold crack include base paper, coating, pigment, latex, humidity and the like. One of the factors that have the greatest impact depends on type of pulp used as the base paper. Fold endurance and fold crack are affected, especially as thickness increases, fold endurance and paper stiffness is increased, but fold crack is significantly worse.
One of the main factors is the presence of a coating layer. Since a coating layer has very high elasticity compared to a base paper layer, presence of the coating layer occur more easily crack and fold cracks because it improved the paper stiffness, but it weakens the strength to withstand the increased surface area of the folded surface that occurs when you bend a paper. Accordingly, even if the paper of the same thickness, the paper having a thicker coating layer is more likely to be vulnerable to much more serious fold crack.
Pigments used in the coating affect fold crack by changing the elasticity of the coating layer. Typically, it is believed that clay compared to calcium carbonate can increase the resistance to rupture fold cracks.
Humidity is also a major factor and when the humidity is lower, flexibility of the paper is deteriorated and fold cracks becomes more severe.
However, despite all of the above factors are important, for example, in case of the coating paper, the based paper or the humidity is easily replace or be difficult to change or control, and thus the most realistic alternative is the characteristics of the coating layer to improve the fold crack.
The most important factors which affect fold crack including elasticity and flexibility and the like of the coating layer is a latex binder. It is a significant relationship between a kind of latex distributed in a coating layer in some way and the fold crack of the coated paper.
In general, it is believed that, as a glass transition temperature of latex used for the coating layer decreases, flexibility of the coating layer is improved and fold crack is thus reduced. In this case, it may be difficult to accomplish the purpose of the protection of package contents, due to severe degradation of paper stiffness.
In addition, when the latex glass transition temperature increase, paper stiffness is increased, but fold crack will be exacerbated. The particle size of the latex may also affect fold crack. As the smaller the particle size of the latex, a large surface area in which the latex is adhered to the pigment increases and thus adhesion force is increased, while the latex tends to be easily swept into the movement of water, and thus the movement and distribution thereof spreads.
In particular, in a case of the latex used for a coating layer adjacent to a base paper layer, as the smaller of the particle size of the latex, the permeation of latex degree into the base paper layer becomes severe, and thus fold crack will affect by changing the characteristics of the base paper layer adjacent to the coating layer.
Taking into consideration these above factors, it is preferable to use paper comprising a thin base paper, a thin coating layer and the latex with a low glass transition temperature to inhibit the fold crack. However, in order to protect the package contents, it is preferable to choose the base paper as thick as possible and thickening of the coating layer gradually increasing the aesthetic sense of paper users is the current trend. Because it may bring a large decrease in the stiffness of the paper, a plan to use the latex having a low glass transition temperature, is not corresponded to one of possible solutions.
Accordingly, there is a need for methods while maintaining paper stiffness as much as possible and reducing fold crack in order to solve these problems.